We have focused primarily on the study of primary effusion lymphoma in patients with AIDS. Primary effusion lymphoma (PEL) is a Kaposi's sarcoma herpes virus (KSHV)-induced lymphoma that typically arises in body cavities of HIV-infected patients. PEL cells are often co-infected with Epstein-Barr virus (EBV). PEL-like lymphoma is a KSHV-unrelated lymphoma that arises in body cavities of HIV-negative patients. PEL-like lymphoma is sometimes EBV-positive. The derivation of PEL/PEL-like cells is unclear. To study PEL pathogenesis, mesothelial cells were cultured from body cavity effusions of 23 patients. These included patients with AIDS with or without PEL presenting with effusions in body cavities, and patients with ovarian carcinoma with malignant or not malignant effusions. Cell proliferation, cytokine secretion, marker phenotypes, KSHV/EBV infection and clonality were evaluated. Gene expression was measured by qPCR and immunoblotting. A mouse model of PEL was used to evaluate tumorigenicity. We found that mesothelia derived from 6 effusions of HIV-infected patients with PEL or other KSHV-associated diseases contain rare KSHV+ or EBV+ mesothelial cells. After extended culture (16-17 weeks), some mesothelial cells underwent a trans-differentiation process generating lymphoid-type CD45+/B220+, CD5+, CD27+, CD43+, CD11c+ and CD3- cells resembling B1-cells, most commonly found in mouse body cavities. These B1-like cells were short-lived. However, long-term KSHV+EBV- and EBV+KSHV- clonal cell lines emerged from mesothelial cultures from two patients, which were clonally distinct from the monoclonal or polyclonal B-cell populations found in the patient's original effusion. The current study provides a novel and unifying insight into PEL and PEL-like lymphomagenesis. Three observations emerged from these studies. First, we discovered that mesothelial cell monolayers undergo a mesothelial-to-lymphoid transition (MLT) resulting in the emergence of lymphoid-type cells. This discovery extends the spectrum of mesothelial functional capabilities, beyond secretion of lubricants, maintenance of surface integrity and ability to repair. MLT resembles the emergence of hematopoietic cells from the endothelium of the yolk sac and the dorsal aorta identified as endothelial-to-hematopoietic transition (EHT). Endothelial and mesothelial cells can undergo phenotypic and functional change through endothelial-to-mesenchymal (EMT) and mesothelial-to-mesenchymal transitions (MMT). The second discovery we made is that the lymphocytes emerging from mesothelial cultures have a B1-like phenotype, supporting a mesothelial origin of human B1-type cells. B1 lymphocytes are the main B-cell population in murine body cavities but are rare elsewhere. Human B1-like lymphocytes have been identified in cord and peripheral blood, but to our knowledge not in body cavities. Despite their importance in immune defense, the origin of B1-type cells is unclear. During mouse development, B1 lymphocytes are first detected at embryonic day (E) 8.0-8.5 in the para-aortic mesoderm prior to the emergence of hematopoietic stem cells (HSC) from the dorsal aorta, suggesting an HSC-independent origin of B1 cells. All mesothelia that line body cavities derive from the para-aortic splanchnopleural mesoderm. Thus, B1-type cells and mesothelia have a common developmental derivation, raising the possibility that persistence of mesodermal precursors within adult mesothelia confers B1-cell differentiation potential to these mesothelia. The third observation we made is that mesothelial cells can be infected by KSHV and EBV. The emergence of monoclonal B-lineage cell lines from mesothelial cultures suggests that PEL and PEL-like lymphoma may derive from KSHV or EBV-infected mesothelial cells. It is noteworthy that two of the 3 lines so derived were clonally distinguishable from the cells originally found in the patient. It remains possible that these lines may have emerged as a result of outgrowth from rare clones of PEL or EBV-infected cells in the original effusion. However, the evidence of MLT transition and clonal analysis suggest that a more likely explanation is that the unique clonal lines derived from the KSHV or EBV-infected mesothelial cells. Interestingly, the indeterminate surface phenotype of PEL cells and cell lines, including the KSHV+ 81 lines, resembles the predominant surface phenotype of the B1-like cells. In addition, the monoclonal KSHV-/EBV+ clone resembles the KSHV- PEL-like lymphoma in showing a mature B-cell phenotype and EBV infection, raising the possibility that the CD20+ cell subset recovered from mesothelial cultures is a source of PEL-like lymphoma. From this perspective, body cavity-associated PEL and PEL-like malignancies would have a common mesothelial derivation. Ongoing and future studies will be focused on determining if this new understanding of PEL pathogenesis can be exploited to prevent PEL development and treat it by targeting the mesothelium.